1. Field of the Invention
The present invention relates to an image forming method wherein development is carried out substantially simultaneously with imagewise exposure from within a photoreceptor, thereby forming a toner image on the photoreceptor, which method is remarkably improved over the conventional Carlson process, is free from evolution of ozone harmful to the human body, and can stably provide a good image at low cost.
2. Description of the Related Art
In recent years, rapid growth of computers and communication technology has led to an ever-increasing demand for printers as output terminals, and electrophotographic printers have rapidly become widespread by virtue of their excellent recording speed, print quality and other properties.
In the conventional electrophotographic system (Carlson process), a photoreceptor is used as a recording medium, and recording is carried out through a series of complicated steps of electrification, exposure, development, transfer, fixation, de-electrification, and cleaning, which steps limit the possible reductions in size and cost, and prevent realization of maintenance-free operation. For this reason, the development of a simpler developing process has been desired in the art. In recent years, attempts to carry out developing using a transparent photoreceptor have been made, and there is a report that a reduction in size can be realized by eliminating the above conventional electrification mechanism and disposing an optical system within a photoreceptor. For example, Japanese Patent Application No. 5-143262 proposes a process wherein an organic photoreceptor is used and developing is carried out with a toner and a carrier.
The principle of this process will now be described.
FIGS. 1 and 2 are diagrams showing the principle of forming an image by the above process. A photoreceptor 1 comprises a transparent substrate 2, a transparent conductive layer 3, and a photoconductive layer 4, and the transparent conductive layer is grounded. A developer 5 comprises a high-resistance carrier 6 and an insulating toner 7. A developing roller 8 comprises a magnet roller 9 and, provided thereon, a conductive sleeve 10. The developer is attracted to the developing roller by magnetic force, deposited on the sleeve and, in this state, carried to the photoreceptor. Within a developing nip, the following three steps are successively carried out instantaneously. Specifically, in a zone (1), the photoreceptor 1 is subjected to electrification 12 through the developer 5. In a zone (2), the electrified photoreceptor 1 is then subjected to imagewise exposure through the transparent substrate 2 to form a latent image. Numeral 11 designates an optical system. Further, development occurs in a zone (3) at its latent image forming portion, because electrical adhesion 13 of the toner 7 to the photoreceptor 1 is higher than magnetic force 14 from the magnet roller 9, electrostatic attractive force from carriers on the magnet roller 9, and mechanical scraping force. Further, in the background other than the latent image forming portion, the toner 7 is recovered by taking advantage of the magnetic force and electrostatic attractive force from the magnet roller 9 and the magnetic carriers and the mechanical scraping force. Therefore, as compared with a nonmagnetic toner, a magnetic toner, by virtue of using magnetic attractive force, is more advantageous as a toner from the viewpoint of the prevention of background fog. Since, however, a nonmagnetic toner can be recovered by taking advantage of electrostatic attractive force from the carriers and the mechanical scraping force, it is also possible to use a nonmagnetic toner. The developed toner is transferred onto a recording medium, that is, paper or a plastic sheet, to provide a print. The above process will be hereinafter referred to as "optical back recording process or system."
The above-described optical back recording system is different from the conventional system (hereinafter referred to as "Carlson system"). As is well known in the art, for the Carlson system, the electrification of a photoreceptor, exposure, and development are carried out by separate processes, enabling the electrification potential of the photoreceptor to be set at a higher value than the developing bias so as not to cause background fog. The toner is carried electrostatically to the latent image, whereas no toner is deposited on the background. On the other hand, for the optical back recording system, since the surface potential of the photoreceptor is created by the developing bias, the potential of the photoreceptor is equal to or, due to a small decrease in efficiency, smaller than the developing bias. Therefore, the toner deposited on the background is recovered by the magnetic or electrostatic attractive force from the magnetic roller and the mechanical scraping force. An enhancement in the recovering capability for the purpose of reducing background fog results in lowered print density. The attainment of a combination of reduced background fog and a high print density is highly sought after in the art.
Further, for the optical back recording system, electrification and development occur substantially simultaneously in the photoreceptor through a developing agent. This necessitates the use of a developing agent having high electrification and development capability. However, when the developing agent disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-15055 is used, the toner concentration margin (which means that satisfactory printing properties can be obtained in a toner concentration of 10 to 30% by weight) is unsatisfactory. Satisfactory printing properties should be obtainable in a toner concentration of 10 to 30% by weight is that demand for reduced cost has led to a tendency for the conventional toner concentration control system using a magnetic sensor to be replaced by an automatic toner concentration control system as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 5-150667. The conventional magnetic sensor can control the toner concentration to any desired value within .+-.2%, whereas the above automatic toner concentration control system can carry out only a rough control of the toner concentration, i.e., to the extent that the toner concentration will fall within a range of 10 to 30% by weight.